Automated shift transmissions designed as group transmissions with a multi-gear main transmission and a downstream group, in particular one designed as a range group drivingly coupled downstream from the main transmission, and sometimes with an upstream group, in particular one designed as a splitter group drivingly coupled upstream from the main transmission, have been known for a long time and are preferably used in utility vehicles. By means of a splitter group of two-stage design with a gear ratio interval that corresponds approximately to half of an average gear ratio interval between two successive gears of the main transmission, the gear intervals of the main transmission are halved and the total number of gears available in the group transmission is doubled. For example, by means of a two-stage range group with a gear interval above the overall gear interval of the main transmission by approximately an average gear interval between two successive gears of the main transmission, the ratio spread of the group transmission is approximately doubled and the total number of gears available is again doubled.
The splitter group can be connected upstream or downstream from the main transmission and can therefore be made as an upstream or a downstream group. Likewise, the range group can be connected upstream or downstream from the main transmission and can therefore be an upstream or a downstream group. Automated shift transmissions, which comprise interlocking shifting elements, are to be distinguished from automatic power shift transmissions with frictional shifting elements.
The sole FIGURE shows a layout of a group transmission CT of the so-termed AS-Tronic family produced by the present applicant together with a drive aggregate A and an axle drive AB. The group transmission CT shown in the FIGURE comprises a main transmission HG, an upstream group in the form of a splitter group GV drivingly coupled upstream from the main transmission HG and a downstream group in the form of a range group GP drivingly coupled downstream from the main transmission HG. The main transmission HG of the group transmission CT in the FIGURE is in the form of a direct-gear transmission of countershaft design and comprises a main shaft WH and two countershafts WVG1 and WVG2, the first countershaft WVG1 being provided with a controllable transmission brake Br.
The main transmission HG is formed as a three-stage transmission with three gears G1, G2 and G3 for forward driving and with one gear R for driving in reverse. Loose wheels of the gears G1, G2 and R are respectively mounted to rotate on the main shaft WH and can be engaged by associated claw clutches. The associated fixed wheels are arranged in a rotationally fixed manner on the countershafts WVG1 and WVG2. The highest gear G3, made as a direct gear, can be engaged by means of a direct shifting clutch. The shifting clutches of the gears G3 and G2 and the shifting clutches of the gears G1 and R are in each case in the form of claw clutches and are respectively combined in common shifting packets S1 and S2.
The upstream group of the group transmission CT in the FIGURE, formed as a splitter group GV, is a two-stage group and is also of countershaft design such that the two gear steps K1 and K2 of the upstream group GV form two shiftable input constants of the main transmission HG. By virtue of a small gear ratio difference between the two gear steps K1 and K2, the upstream group GV constitutes a splitter group.
The loose wheel of the first gear step K1 is mounted to rotate on the input shaft WGE, which is connected by way of a controllable separator clutch K to the drive aggregate A indicated by broken lines, which is in the form of an internal combustion engine and is not part of the group transmission.
The loose wheel of the second gear step K2 is mounted to rotate on the main shaft WH. The fixed wheels of the two gears K1 and K2 of the upstream group GV are respectively arranged in a rotationally fixed manner on the countershafts WVG1 and WVG2 of the main transmission HG, in each case extended on the input side. The synchronized shifting clutches of the upstream group GV, formed as claw clutches, are combined in a common shifting packet SV.
The downstream group of the group transmission CT in the FIGURE, arranged after the main transmission HG and formed as a range group GP, is also of two-stage design but is of planetary configuration with a simple planetary gearset. The sun gear PS is connected in a rotationally fixed manner to the main shaft WH of the main transmission HG extended on the output side. The planetary carrier PT is coupled in a rotationally fixed manner to the output shaft WGA of the group transmission CT, which is connected to an axle drive AB indicated by a broken line. The ring gear PH is connected to a shifting packet SP with two synchronized shifting clutches in the form of claw clutches by means of which the range group GP can be shifted alternatively, by connecting the ring gear PH to a fixed housing component to produce a slow-driving stage L or, by connecting the ring gear PH to the main shaft WH or the sun gear PS, to produce a fast-driving stage S. The range group GP can be shifted with synchronization.
Power can be branched off from the countershaft WVG2 of the group transmission CT to drive a power take-off PTO.
Such a group transmission is known for example from DE 10 2007 010 829 A1, wherein this prior art also shows a group transmission with a four-stage main transmission.
The main transmission HG of such a group transmission CT is made as an unsynchronized main transmission, while in contrast the downstream group that forms the range group GP and the upstream group that forms the splitter group GV are designed as synchronized transmission components. In this case, when carrying out a shifting operation, depending on the existing rotational speed difference, the transmission input or input shaft WGE of the group transmission CT has to be accelerated or braked to a required rotational speed, the braking taking place with the help of the transmission brake Br and the acceleration by controlled action upon the drive aggregate A with the separator clutch K engaged. For this synchronization process, the main transmission is then brought to a defined neutral position.
Now if, in a vehicle that is being driven and whose main transmission HG in the group transmission CT is in the neutral position, a gear is again to be engaged in the main transmission or the group transmission, then to do this it is as a rule necessary for the drive aggregate A to provide some assistance. In such a case, namely due to the speed of the moving vehicle, the output shaft WGA of the group transmission CT is being rotationally driven so that to produce a synchronized condition, the transmission input shaft WGE also has to be driven.
But if the drive aggregate A is malfunctioning, for example if it has stopped or can no longer be controlled, then the drive aggregate A can no longer provide any motor assistance for producing the synchronous condition, so that according to the prior art it is no longer possible to engage a gear.
Accordingly, a method for operating a drive-train with a group transmission is needed, with the help of which, when a vehicle is being driven and its main transmission HG is in the neutral position, a gear can be engaged in the group transmission CT even without motor assistance from the drive aggregate A.